After a computer system is turned on or reset, many electronic devices in the computer system need to be driven to an initial condition prior to being used. An example of such an electronic device is a memory device. A typical memory device has memory cells which use a variety of logic circuits such as latches, gates and flip-flops to support their operation. These logic circuits need to be driven to an initial condition before the memory device is used to store data. A typical memory device also has a variety of capacitively loaded nodes which need to be driven to an initial condition such as a particular voltage.
Many electronic devices need a certain amount of time to enter an initial condition before they are ready to be used. One such electronic device is a memory device referred to as a dynamic random access memory (DRAM). A typical DRAM has a variety of capacitively loaded nodes which need to be driven to an initial condition at a particular voltage. For example, a typical DRAM has a plurality of DRAM memory cells, each of which includes a capacitor and an NMOS transistor. Each capacitor in each DRAM memory cell has one plate of its own coupled to a source terminal of its respective NMOS transistor and another plate shared in common among the capacitors in all of the DRAM memory cells. This common capacitor plate is referred to as the cell plate, and it often needs to be driven to an initial condition at a particular voltage. Since the cell plate is capacitively loaded, it is a node which needs a certain amount of time to enter the initial condition at the particular voltage. Since the typical DRAM has a variety of capacitively loaded nodes, including the cell plate, it has an overall RC time constant which controls the amount of time it needs to enter the initial condition. In a typical DRAM, this amount of time is in the range of 50 .mu.S.
Since many electronic devices need to be driven to an initial condition prior to their use, circuit designers have conventionally designed these electronic devices to respond to a signal referred to as a power-up signal by entering the initial condition. Also, since these electronic devices often need a certain amount of time to enter the initial condition, circuit designers have conventionally used a resistor-capacitor (RC) time delay circuit to provide the power-up signal. In a typical RC time delay circuit, a voltage on a capacitor in an RC network in the circuit begins rising as a function of an RC time constant of the RC network when the circuit detects a supply voltage rising from a quiescent voltage such as 0.0 volts. This rising supply voltage is indicative of a computer system which has just been turned on or reset. At the same time that the voltage on the capacitor is rising, the RC time delay circuit provides the power-up signal to an electronic device to drive the electronic device to an initial condition. When the voltage on the capacitor rises to a certain level, the RC time delay circuit trips and stops providing the power-up signal to the electronic device. Circuit designers set the RC time constant of the RC network such that the RC time delay circuit provides the power-up signal to the electronic device for at least the amount of time the electronic device needs to enter the initial condition.
Although the conventional RC time delay circuit is generally adequate for driving an electronic device into an initial condition prior to its use, the present invention recognizes that the circuit is problematic for other reasons.
The conventional RC time delay circuit cannot adequately detect a transient, such as a negative glitch, in a supply voltage being provided to an electronic device because the RC time constant of the RC network of the circuit makes the circuit too slow to react to the transient. This is a problem because, in many of the electronic devices the RC time delay circuit is used with, a transient in the supply voltage can cause the electronic device to enter an unknown state. In the case of one typical DRAM, for example, a transient in the supply voltage which drops the supply voltage below a threshold voltage of about 1.5 volts can cause a variety of latches, gates and flip-flops used to support the operation of memory cells in the DRAM to unlatch and lose latch data they are storing. This problem is exacerbated by a modern trend toward reducing power consumption in electronic devices by using lower supply voltages such as 3.3 volts instead of higher supply voltages such as 5.0 volts. With a lower supply voltage, a transient in the supply voltage is more likely to cause an electronic device to enter an unknown state.
Therefore, there is a need in the art for a circuit which drives an electronic device to an initial condition prior to its use and which drives an electronic device to an initial condition upon detecting a transient in a supply voltage being provided to the electronic device.